Hydroclimatic flood trends in the northeastern United States. Matt Collins, P.H. NOAA Restoration Center

Transcription

1 Hydroclimatic flood trends in the northeastern United States and implications for planning and design Matt Collins, P.H. NOAA Restoration Center Gloucester, MA

2 Lake Champlain, VT, April - May, 2011 Credit: LCBP

3 Route 4 near Mendon, VT, August, 2011 Credit: Lars Gange and Mansfield Heliflight

4 Ipswich River, Ipswich, MA, May, 2006

5 Shawsheen River, Andover, MA, April, 2007 Credit: Joseph Zanca, USGS

6 Warwick, RI, March, 2010 Credit: AP photo/joe Giblin

7 Black River, Springfield, VT, November, Credit: AP photo/vhs

8 So is it just a perception that we are seeing more and larger floods in recent years? Wh t i th id f h d li ti i i What is the evidence for hydroclimatic increases in flood magnitudes and/or frequencies in this region?

9 Presentation map Briefly review recent precipitation trend research for Northeast U.S. Present results from three flood trend studies: trends in New England flood magnitudes and implications for flood risk estimates (Collins, 2009); trends in New England flood frequency (Armstrong et al., 2012); trends in Mid-Atlantic flood magnitude and frequency (Armstrong et al., in review) Discuss implications for flood risk planning and design Very briefly discuss relationships between flood trends and large-scale atmospheric circulation patterns

10 Recent precipitation research Karl and Knight (1998) and Madsen and Figdor (2007) document increased precipitation nationally over last 100 years Primarily through heavy and extreme events Pronounced in Northeast U.S. Recent studies focused on Northeast U.S. extreme precipitation corroborate these results (Douglas and Fairbank, 2011; Spierre and Wake, 2010)

11 Flood trend study area

12 Methods Using data from U.S. Geological Survey (USGS) stream gauges: Annual maximum series for magnitude studies Peaks-over-threshold series for frequency studies

13 annual maximum 13 peaks-over-threshold (POT/WY) Streamflow (m 3 /s) annual maximum series (AMS) Q = 25.5 m 3 /s

14 Methods Avoiding gauges in developing (urbanizing) watersheds Using USGS Hydro-Climatic Data Network (HCDN) relatively free of human influence nearly 250 in HUCs 01 and 02 Study criteria for HCDN gauges: records beginning by at least 1951 continuous through 2006 (HUC 01) or 2009 (HUC 02) no evidence of flow regulation impacting flood flows

15 Study gauges 103 AMS records 90 POT/WY records Average record ~75 years longest record ~ 100 years

16 Methods Mann-Kendall non-parametric test for monotonic trends (simply or with time) Wilcoxon rank-sum test to investigate stepped trends Flood risk analyses following federal interagency guidelines ( Bulletin 17B ) (IACWD, 1981)

17 Flood magnitude results 78 of 103 gauges (76%) show flood magnitude over time 25 (24%) have p<0.1 after Collins (2009) and Armstrong et al. (in review)

18 Step changes in magnitude? St. John River at Ninemile Bridge, Maine (USGS Gage No ) Z Sco ore

19 Step changes in flood magnitude? Wilcoxon rank-sum test Comparison groups: post-1970 Z scores pre-1970 Z scores 90 of 103 (87%) gauges have post-1970 > pre of 103 (39%) have p<0.1

20 Flood frequency results 72 of 90 gauges (80%) show POT/WY over time 32 (36%) have p<0.1 after Armstrong et al. (2012) and Armstrong et al. (in review)

21 Step changes in frequency post-1970 > pre-1970 at 82 of 90 gauges (50 have p<0.1) median increase of 1 flood p/year post-1970 in New England after Armstrong et al. (2012)

22 Flood risk estimates What happens to flood risk estimates when there are stepped, upward trends in the annual flood series? In Northeast U.S., dated estimates will usually be underestimates t (e.g. older FEMA Flood Insurance Studies)

23 North River at Shattuckville, MA (USGS Gage No ) 1000 log-pearson Type III ( ) floods log-pearson Type III ( ) floods log-pearson Type III ( ) Discharge (m 3 /s) floods Annual Exceedance Probability bilit (percent)

24 Mount Hope River near Warrenville, CT (USGS Gage No ) 1000 log-pearson Type III ( ) floods log-pearson Type III ( ) floods log-pearson Type III ( ) Discharge (m 3 /s) floods Annual Exceedance Probability bilit (percent)

25 Large-scale atmospheric circulation We ve found statistical relationships between the North Atlantic Oscillation (NAO) and our flood data NAO: One of the oldest known, prominent, and recurrent upper atmospheric circulation patterns Alternate modes strongly influence climate variability along the U.S. east coast, especially in winter Dominantly positive (wet) mode since ~1970 NAO is an expression of natural variability in atmosphere, but some indications it is being/will be affected by global warming

26 Summary Larger and more frequent floods over last decade appear to be part of a hydroclimatic step increase that began around 1970 Trends especially compelling given rural NE land cover trends over last 100 years NAO phase is broadly synchronous with step change in flood magnitude and frequency, merits further research Step changes in flood series should be considered when estimating flood risk for NE watersheds

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28 NAO Positive NAO warmer, wetter NE winters Negative NAO colder, drier NE winters Bradbury et al. (2002)

29 PC-Based NAO Index (Dec-Mar) Z- -score NAO and NE Floods Z score No correlation annually Significant Average Standardized Departures for 23 New England Flood Series 2 correlation if floods lagged by 1 year 1.5 (p<0.1) Substantial decadal 0 correspondence annual series